The sample, sealed in a two-inch-diameter glass container, was bought by a Cheltenham-based autograph collector on behalf of an American firm.
Believed to have come from the original penicillium notatum mould, its underside was signed by the discoverer of penicillin, Alexander Fleming, a year before his death. It reads: "Penicillium notatum - the mould that makes penicillin. Alexander Fleming, 1954."
Bidding at the London sale for the item, described by Christie's auctioneer and medical expert Jeremy Collins as "an extremely important part of medical history", was tense. Starting at pounds 1,900, the offers came flooding in until only two heavyweight bidders remained.
A mystery telephone bidder offered pounds 6,500 before a representative of John Wilson Autographs Ltd in Cheltenham came in with a bid of pounds 7,000 to take the medical souvenir. When the buyer's premium of 15 per cent is added, the price is pounds 8,050.
The representative said: "I believe that there are other pieces of penicillin but I am absolutely thrilled to have bought this piece." Mr Collins thought that only five pieces had been sold in the past 20 years and that "any museum which has a serious collection should have one".
Penicillin was discovered by Fleming, a Scotsman, in 1929 and was ready for commercial use by 1940. The development of the drug saved the lives of thousands of soldiers during the Second World War and Fleming, with two colleagues, went on to share the Nobel Prize for Medicine in 1945.Reuse content